1. Field of the Invention
This invention relates to steam turbines and, more particularly, to an apparatus and method for incorporating a drain cooler in a multiple drain receiver reheat system.
2. Description of the Prior Art
Virtually all nuclear steam turbine generators, operating under slightly wet or low superheated initial steam conditions, incorporate steam-to-steam reheat to improve thermal performance and reduce blade erosion. Rising fuel costs have led to the use of higher initial operating pressures and temperatures and additional reheat features, including an increase in the number of heaters that are employed in a turbine cycle. The higher pressures and temperatures have led to other design developments including provision for higher outlet water temperatures by utilizing superheat of the steam, and drain cooling sections in the heaters that subcool condensate.
In some prior applications of steam-to-steam reheater drains, drain fluid is discharged as a mixture of condensed steam and scavenging steam from a high pressure reheater in a moisture-separator-reheater (hereinafter MSR) to the highest pressure feedwater heater where the fluid is combined with condensed heater steam from a first turbine extraction point. ("Scavenging steam" refers to small amounts of dry steam bled from the main steam supply lines and directed through the tubes of the reheater bundle to prevent the condensate from subcooling and collecting, particularly in those tubes at the lower elevations of the bundle or the outermost U-shaped tubes of the bundle which are exposed to the lowest temperature incoming steam to be reheated. Condensate collection may result in subcooling and the associated sudden temperature change may damage piping when condensate is eventually blown from the piping by the pressure build-up. Steam-to-steam reheat designs usually employ approximately 2% of total reheater steam supply at rated load for scavenging steam to prevent moisture build-up in the reheater tubes.)
From the highest pressure feedwater heater, the condensed steam and other drain flows are then discharged or cascaded seriatim to lower and lower pressure feedwater heaters until at some point in the cycle, the flows become part of the main feedwater stream.
As previously disclosed in U.S. Pat. No. 4,825,657 assigned to Westinghouse Electric Corporation, the drains leaving the MSR high pressure reheater are considerably hotter than the feedwater leaving the highest pressure feedwater heater, as much as 55.degree. C. (100.degree. F.) at rated load, and in excess of 140.degree. C. (250.degree. F.) at 25% load. Accordingly, the drains must be throttled down to the feedwater pressure prior to heat exchange. This results in a loss in thermal efficiency.
One suggested method of minimizing this loss is to pump the high pressure reheater drain fluid into the outlet of the highest pressure feedwater heater. Major drawbacks of this method are: a) an additional pump is required; b) the difficulty of avoiding cavitation due either to insufficient net positive suction head in steady state conditions or to flashing during transients; and c) disposal of scavenging steam that is used to enhance the reheater tube bundle reliability.
The above-referenced U.S. Pat. No. 4,825,657 describes a method and apparatus for improving the thermal efficiency of steam-to-steam reheating systems within steam turbine generator systems by allowing the reheater drain fluid to be directly added to the feedwater stream without the need for additional pumping through use of a drain cooler. The high pressure reheater drain fluid passes through the drain cooler in heat exchange relationship with condensate from the discharge of the highest pressure feedwater heater. This avoids the loss of thermal efficiency resulting from throttling of the reheater drain pressure. Heat rate improvement is greater when the system is operated at less than 100% load. The disclosed system is set forth in the context of field retrofit application to single and multi-stage moisture-separator-reheaters. These existing systems include drain receivers with level controls. Fluid from high pressure reheater drains is collected in the drain receivers and then directed to a heat exchanger (drain cooler) in heat exchange relationship with condensate from a high pressure feedwater heater. The use of a drain cooler avoids loss of thermal efficiency from throttling of reheater drain pressure.
Conventional reheater drain systems customarily employ a pressure breakdown section between the MSR reheater drain connection and the feedwater heater receiving the drain fluid, and a level-controlled drain receiver to accept the condensed heating steam. There is a significant reliability problem with drain receivers, which frequently produce internal flooding in the drain tube bundle from the high pressure MSR. Such flooding has contributed to numerous damaged tube bundles, necessitating reduced load operation at impaired plant efficiency.
Further, because of the decrease in heater pressure at low loads, accompanied in many instances with an increase in reheater supply pressure, the percentage of scavenging steam increases with decreasing load. However, an increase in scavenging steam has been shown to have only a small affect on the heat rate of a cycle employing a drain cooler.
U.S. Pat. No. (55,161) discloses a method and apparatus for improving a steam-to-steam reheat system in a steam turbine employing a drain cooler. The utility of a drain cooler is enhanced by installing a condensate bypass line with a control valve to allow adjustment of the condensing capability of the drain cooler by optimizing the amount of scavenging steam in accordance with load conditions, thereby achieving a heat rate reduction. A steam turbine generator employs a steam-to-steam reheating system which utilizes a small component of scavenging steam to prevent moisture build-up in the bottom most tubes of a reheater bundle. The system has a high pressure moisture-separator-reheater with a reheater drain, and several feedwater heaters connected in series to heat feedwater of increasing pressure. Each of the feedwater heaters has an inlet and an outlet for feedwater. Heating of feedwater is accomplished in a drain cooler which receives fluid from the reheater drain and passes it in heat exchange relationship with outlet feedwater prior to feeding the reheater drain fluid to the highest pressure feedwater heater. The system controls the amount of scavenging steam and the fluid level at the drain cooler heat exchanger to control the heat capacity of the drain cooler and eliminate the need for a drain receiver level control.
In most existing utility power plant installations, the reheat system employs a plurality of moisture-separator-reheaters (MSR). The separate MSR's or even the separated drains from a common MSR may discharge at different pressures. Each drain is typically directed to a corresponding drain receiver and each drain receiver includes a control valve for maintaining a preselected liquid level in the respective drain receiver. The liquid acts as a seal between the higher pressure steam within the MSR tube bundle and the lower pressure feedwater heater line.
It is desirable to combine the drain lines from each drain receiver into a manifold so that a single line connects to a drain cooler. However, the normal pressure variation between drain receivers may be from 703 k/m.sup.2 to 17577 k/m.sup.2 (one to twenty-five pounds per square inch (PSI)). This difference in pressure could force condensate liquid from one of the lower pressure drain receivers back into the associated MSR and result in internal flooding. Such flooding is believed to be the cause of various turbine performance and reliability problems.